Monitoring river island dynamics using aerial photographs and lidar data: the tagliamento river study case

Abstract

River islands are defined as discrete areas of woodland vegetation located in the riverbed and surrounded by either water-filled channels or exposed gravel. To be defined as such, they also have to exhibit stability over time and remain exposed during bank-full flows. River islands are very important from both a morphological and an ecological point of view, as they represent the most natural state of a fluvial system and are to a large degree influenced by human pressure. This study aims to analyze morphological and vegetation characteristics of three types of river islands, pioneer, young and stable, and their dynamics after different flood events in the Tagliamento River, a natural gravel bed river with highly natural conditions. The Tagliamento River, with a length of 178 km, is the main river of the Friuli-Venezia Giulia region, and it represents a link between Alpine and Adriatic zones. Because of the limited human pressure, the main course of Tagliamento is characterized by highly complex fluvial dynamics. The analyses were carried out in two sub-reaches characterized by different dominant morphology but equally low human pressure. Island dynamics, in relation with flood events, have been analyzed using aerial photos taken over two consecutive years, 2010 and 2011. Mean elevation of surface and maximum height of vegetation were estimated for each river island using digital models obtained from two LiDAR datasets. The results suggest that river islands lie at different elevations, and this implies a different resistance capacity during flood events. Pioneer river islands and young river islands lie at lower elevations than stable islands causing a total or partial incapacity to survive during considerable flood events. In most cases, the islands’ typology (pioneer, young, or stable) is cancelled by ordinary floods. Stable islands lie at higher elevations, and only intense and infrequent flood events, i.e. recurrence interval (RI) >10–15 years, are able to determine substantial erosions. The presence of high vegetation, in some of our study cases higher than 30 m, contributes to increase the resistance of stable islands. River islands are very dynamic elements, strictly associated with the occurrence of major floods and to the morphological characteristics of the fluvial system. The main morphological changes have been documented in the braided sub-reach; the highly dynamic response to changes of the fluvial reach also affects the islands stability. However, the different resistance capacity during flood events of pioneer, young, and stable islands as well as the different bank erosion capacity are important aspects to better understand dynamics of fluvial islands. In this sense, the study could be helpful to predict the hydraulic hazard linked to the morphological changes and to formulate more effective flood management programs.